Imprinting Evolved Over 125 Million Years Ago

9 January 2004: Genomic imprinting involves the epigenetic inactivation of one allele of a gene, resulting in monoallelic expression that is parent-of-origin dependent (Murphy and Jirtle BioEssays 25: 577-588, 2003). Metatherians (opossum) and Eutherians (humans and mice) are imprinted, whereas the egg-laying Prototherian mammals (platypus and echidna) lack imprinted genes (Killian et al. Mol. Cell 5: 707-716, 2000). These findings are most parsimonious with genomic imprinting evolving in a common ancestor to the Therian mammals whose offspring are born alive.

Recently, Luo and his colleagues (Science 302: 1934-1940, 2003) found a 125 million year old marsupial-like mammalian fossil (Sinodelphys szalayi) in the Yixian Formation of China. This important discovery extends the record of marsupial relatives with skeletal remains by 50 million years to the same age as the oldest known eutherian mammal, Eomaia (Ji et al. Nature 416: 816-822, 2002). Moreover, molecular analyses of amino acid sets of the M6P/IGF2R and BRCA1 proteins indicate the Metatherian / Eutherian split occurred 182 to 190 million years ago (Woodburne et al. Mol. Phylogenet. Evol. 28: 360-385, 2003). Together these findings demonstrate that imprinting evolved more than 125 million years ago during the Late Jurassic / Early Cretaceous period.